A number of processes including calcium "pulses", mitochondrial dynamics, mitophagy, and mitochondrial biogenesis and repair sustain healthy mitochondria in the cell. A failure in any of these functions of "healthy" mitochondria can promote mitochondrial dysfunction and the accumulation of damaged mitochondria and cellular dysfunction. Reactive oxygen species and mitochondrial permeability transition are some of the effects of "sick" mitochondria. 

Featured in the Cell Press Supplement - Cell Press Selections: Mitochondria.